Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). They can occur in either chamber of the heart (i.e., ventricles or atria) or both. Examples of tachyarrhythmias include sinus tachycardia, ventricular tachycardia, ventricular fibrillation (VF), atrial tachycardia, and atrial fibrillation (AF). Tachycardia is characterized by a rapid rate but with an orderly contraction of the heart chamber, either due to an ectopic excitatory focus or abnormal excitation by normal pacemaker tissue. Fibrillation occurs when the chamber depolarizes in a chaotic fashion with abnormal depolarization waveforms as reflected by an EKG. An electrical shock applied to a heart chamber (i.e., defibrillation or cardioversion) can be used to terminate most tachyarrhythmias by depolarizing all excitable myocardium of the chamber, which thereby prolongs refractoriness, interrupts reentrant circuits, and discharges excitatory foci. Implantable cardioverter/defibrillators (ICDs) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device. An ICD is a computerized device containing a pulse generator that is usually implanted into the chest or abdominal wall. Electrodes connected by leads to the ICD are placed on the heart, or passed transvenously into the heart, to sense cardiac activity and to conduct the impulses from the pulse generator. Typically, the leads have electrically conductive coils along their length that act as electrodes. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, by delivering a shock pulse that impresses an electric field between the electrodes to which the pulse generator terminals are connected.
The most dangerous tachyarrhythmias are ventricular tachycardia and ventricular fibrillation, and implantable cardioverter/defibrillators (ICDs) have most commonly been applied in the treatment of those conditions. ICDs are also capable, however, of detecting atrial fibrillation and delivering a shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrioventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, ICDs configured to treat AF offer a number of advantages to certain patients, including convenience and greater efficacy.
ICDs for delivering ventricular defibrillation shocks typically use a capacitor that is charged from a battery with an inductive boost converter to deliver the shock pulse. When ventricular fibrillation is detected, the ICD charges up the capacitor to a predetermined value for delivering a shock pulse of sufficient magnitude to convert the fibrillation (i.e., the defibrillation threshold). The capacitor is then connected to the shock electrodes disposed in the heart to deliver the shock pulse. Since ventricular fibrillation is immediately life threatening, these steps are performed in rapid sequence with the shock pulse delivered as soon as possible. Similarly, an ICD for treating atrial fibrillation charges an energy storage capacitor prior to delivering an atrial shock pulse once atrial fibrillation is detected. Additional constraints are presented in this situation, however. First, there is the risk that an atrial shock pulse can actually induce ventricular fibrillation, a condition much worse than atrial fibrillation. To lessen this risk, the ICD must ensure that the ventricular rhythm is regular and wait for an appropriate time window with respect to sensed ventricular depolarizations in which to deliver the atrial shock pulse. During this waiting period, the voltage of the energy storage capacitor begins to decay such that, in a typical electrolytic capacitor used in ICDs, the capacitor voltage can fall by 5% in 10 seconds. If the waiting period is long enough, the capacitor voltage may actually fall below the value necessary to deliver a shock pulse above the defibrillation threshold. It is this problem with which the present invention is primarily concerned.